Better error message when gdbserver fails on LD_PRELOAD

[disconnect3d]

This is a fix for #1069. I have added a deeper description "what is going on under the hood" below.

When one passes a env={'LD_PRELOAD': '...'} to gdb.debug which cannot be preloaded the process_created string in _gdbserver_port might end up looking like this:

"ERROR: ld.so: object '/bin/bash' from LD_PRELOAD cannot be preloaded (cannot dynamically load executable): ignored.\n"

And this makes so that pwndbg ends up showing an unintuitive error:

$ python -c 'from pwn import *; gdb.debug("/bin/ls", env={"LD_PRELOAD": "/bin/bash"})'
[+] Starting local process '/usr/bin/gdbserver': pid 20691
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "pwnlib/context/__init__.py", line 1349, in setter
    return function(*a, **kw)
  File "pwnlib/gdb.py", line 397, in debug
    port = _gdbserver_port(gdbserver, ssh)
  File "pwnlib/gdb.py", line 234, in _gdbserver_port
    gdbserver.pid   = int(process_created.split()[-1], 0)
ValueError: invalid literal for int() with base 0: 'ignored.'

This patch adds a special check whether the process_created string starts with 'ERROR:'. If so, it raises an exception, so things are more clear for the user:

$ python -c 'from pwn import *; gdb.debug("/bin/ls", env={"LD_PRELOAD": "/bin/bash"})'
[+] Starting local process '/usr/bin/gdbserver': pid 20863
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "pwnlib/context/__init__.py", line 1349, in setter
    return function(*a, **kw)
  File "pwnlib/gdb.py", line 402, in debug
    port = _gdbserver_port(gdbserver, ssh)
  File "pwnlib/gdb.py", line 236, in _gdbserver_port
    'Failed to spawn process under gdbserver. gdbserver error message: %s' % process_created
ValueError: Failed to spawn process under gdbserver. gdbserver error message: ERROR: ld.so: object '/bin/bash' from LD_PRELOAD cannot be preloaded (cannot dynamically load executable): ignored.

[*] Stopped process '/bin/ls' (pid 20863)

I assume this is rather an enhancement instead of a super important hotfix, so I am targeting dev branch for now.
If you want, either switch it or tell me to do so (if it cannot be done automatically).